The present invention relates to an optical recording/reproducing apparatus and more particularly to an optical recording/reproducing apparatus for recording and reproducing data out of an overwritable optical recording/reproducing medium, e.g., a phase change optical medium.
The prerequisite for an overwritable optical recording/reproducing medium, particularly a phase change optical medium, is that in the event of overwriting, an optical head outputs both the erasing power and the recording power in a stable manner. Accordingly, when erasing power and/or recording power differ from optimal power, record marks formed on the recording film of the medium deteriorates and thereby increasing a record/reproduction error rate.
A conventional optical recording/reproducing apparatus includes a mechanism to maintain the mean output of an optical head constant. This function corresponds to maintaining recording power to be constant at the time of overwriting. As for erasing power, while the minimum value in the binary modulation condition of an optical head must be constantly maintained, it has been customary to fix a current that sets the mimimum value.
A conventional optical recording/reproducing apparatus will be described with reference to FIG. 4. As shown, the apparatus includes a monitor sensor 110, an amplifier 120, a comparator 130, current amplifiers 140 and 170 each for setting an amount of laser diode emission, a reference recording power value generator 150, a reference erasing power value generator 160, and a laser diode 180 playing the role of an optical head.
In operation, the monitor sensor 110 senses the amount of emission from the laser diode or optical head 180 during recording. The output signal of the monitor sensor 110 is input to the amplifier 120 and amplified to a DC level necessary for control thereby. An amplified signal output from the amplifier 120 is then fed to one input of the comparator 130. A reference recording power value output from the reference recording power value generator 150 is applied to the other input of the comparator 130. The comparator 130 compares the amplified signal and the reference recording power value and delivers the result of the comparison to the current amplifier 140. Binary record data are also input to the current amplifier 140. A current set by the current amplifier 140 is output when the record data is, e.g., (logical) ONE xe2x80x9c1xe2x80x9d, but it is not output when the record data is (logical) ZERO xe2x80x9c0xe2x80x9d. The current set by the current amplifier 140 is next fed to the laser diode 180.
Also, the reference erasing power value generator 160 feeds a constant current to the current amplifier 170. A current set by the current amplifier 170 is also input to the laser diode 180. In this manner, recording power and erasing power necessary for overwriting are delivered to the laser diode or optical head 180.
As stated above, the conventional apparatus senses the means output of the laser diode 180, compares it with a reference value , and sets a laser diode current or recording power such that the mean output coincides with the reference value. This is adequate to maintain the recording power constant even when temperatures around the apparatus varies. On the other hand, to set erasing power, a constant current is input to the laser diode 180. However, as shown in FIG. 5, the current-to-emission characteristic of the laser diode 180 is such that while a constant current is fed, the elevation of the ambient temperature causes the emission of the laser diode to decrease. Specifically, FIG. 5 indicates that as the ambient temperature rises from 0 C. to 25 C. and further to 50 C. while a constant current is fed, the emission of the laser diode sequentially decreases. Consequently, when the ambient temperature varies, adequate erasing power cannot be set and results in the deterioration of the record/reproduction error rate.
Japanese Patent Laid-Open Publication No. 6-28676, for example, discloses circuitry for controlling not only recording power but also erasing power. The circuitry includes a pin photodiode for sensing the output of a semiconductor laser and sets erasing power and recording power on the basis of the output of the pin photodiode. A sample-and-hold circuit holds the set erasing power and recording power and continuously applies them to the semiconductor laser until the end of the recording and erasing operation. With this configuration, the circuitry may output adequate erasing power just after the start of a recording operation. However, the circuitry does not guarantee that the erasing power remains adequate despite the variation of ambient temperature that may occur after the start of recording.
Japanese Patent Laid-Open Publication Nos. 4-362525, 7-262560 and 10-134352 also teach technologies of the kind described. None of them, however, teaches a method for maintaining adequate erasing power against the varying ambient temperature.
It is therefore an object of the present invention to provide an optical recording/reproducing apparatus capable of maintaining adequate recording power as well as adequate erasing power necessary for recording even when the ambient temperature varies.
An optical recording/reproducing apparatus of the present invention includes a mean value detecting device for detecting a mean value in the binary modulation condition of the output of an optical head, a minimum value detecting device for detecting a minimum value in the binary modulation condition of the output of the optical head, and an input current setting device for setting a current to be input to the optical head in accordance with the above mean value and minimum value.